This invention relates to a method improving the corrosion properties of chromium-alloyed steels.
The alloying of, for example, chromium, molybdenum, silicon or aluminium into steel in order to improve the corrosion properties of the steel involves substantial manufacturing costs. The corrosion properties of a steel being determined by the condition of the steel surface, it is of interest to enrich the steel surface in respect of certain alloying components, which have a positive effect on the corrosion resistance of the steel. In this way, either the amount of alloying components in the steel matrix could be decreased in order to achieve a certain corrosion resistance, or the corrosion resistance for a given composition in the steel matrix could be increased.
For obtaining a surface layer on a substrate where the structure and composition in the surface layer differs from the substrate, one can choose between a number of known methods, for example metallization, electrolytic precipitation or cladding. The principle here is to apply the desired surface layer externally to the substrate via, for example, adsorption, condensation or precipitation on the surface of the substrate from a gaseous or liquid phase.
According to the present invention a surface layer with substantially improved corrosion properties is obtained thereby that, distinguished from the methods mentioned above, an oxide is formed on the surface of the substrate where the oxidized material is comprised in the substrate.
The present invention, thus, relates to a method of making a surface layer with improved corrosion properties on articles of iron-chromium alloys where the article is heated in an oxygen containing atmosphere with vacuum.
The invention is characterized in that the article is heated in an oxygen containing gaseous atmosphere with a total vacuum of about 10.sup.-1 down to 10.sup.-8 mm Hg, preferably about 10.sup.-5 mm Hg, to a temperature, at which diffusion of the most reactive alloying component or components, primarily chromium, with respect to its tendency of combining with oxygen, to the surface becomes perceptible, and at which temperature the remaining properties of the alloy substantially are maintained, and that the described condition for the article is maintained until a surface layer containing oxidized chromium has grown to a thickness of preferably 10.sup.-9 -16.sup.-7 m, whereby the resulting surface layer with respect to structure and composition transforms by degrees from a condition on the surface to a condition in the matrix of the metal alloy.
This invention, thus, distinguished from conventional methods where in all cases a surface layer is applied externally to a substrate, relates to a method, at which the surface layer is made by means of alloying components comprised in the substrate. The surface layer thereby is formed by heating the substrate, which is a chromium steel, in a gaseous atmosphere at vacuum containing a low and well-controlled oxygen content.
At heating in gaseous atmosphere with vacuum, a preferential enrichment to the surface of the chromium steel takes place of that alloying component or those alloying components, which have the strongest tendency of forming compounds with oxygen present on the surface of the chromium steel. The term oxygen present on the steel surface also includes the oxygen comprised in the very thin oxide film, which usually exists on the chromium steel surface prior to the heating in the gaseous atmosphere, and the small but very well controlled amount of oxygen, which exists in the gaseous atmosphere and reaches to the surface of the article.
The temperature, to which the article is heated, is chosen so that diffusion of the most reactive alloying component or components to the surface becomes perceptible. For iron-chromium alloys the diffusion takes place above a temperature of about 300.degree. C. The temperature also must be chosen so that other properties characteristic of the metal alloy are not deteriorated. This temperature is for iron-chromium alloys below about 600.degree. C., because above said temperature carbide is precipitated in the grain boundaries, and in certain cases phase transformations can occur. The stated lower as well as the stated higher temperature, of course, are individual for each type of iron-chromium alloy, but a suitable heating temperature for the majority of iron-chromium alloys is between 300.degree. C. and 600.degree. C. A preferred temperature is about 500.degree. C. Surface layers, thus, with improved corrosion properties have been observed after heating in gaseous atmosphere in the range 300.degree. C.-550.degree. C., with optimum results about 475.degree. C.
The amount of oxygen being present on the surface of the article is to be adjusted both to the heating temperature and to the desired properties of the surface layer. In many applications the oxygen amount level should be so low that substantially only one of the alloying components of the article is capable to react with oxygen. For effecting such a low supply of oxygen to the article surface, normally vacuum conditions are required, which depending on metal alloy and surface layer can extend from tenths of millimeters Hg to ultra-high vacuum conditions (&lt;10.sup.-8 mm Hg). The heating can in certain cases also be carried out in protective atmosphere, but then the difficulties of maintaining a sufficiently low and well-controlled oxygen level are more obvious.
For obtaining a good result, said pressure range implies a total vacuum of about 10.sup.-2 -10.sup.-8 mm Hg. A preferred pressure is about 10.sup.-5 mm Hg.
According to the invention, the article is to be treated at said pressure and temperature until a surface layer containing oxidized chromium has grown to a thickness of about 10.sup.-16 -10.sup.-6 m and preferably 10.sup.-9 -10.sup.-7 m, which, thus, is an extremely thin surface layer.
The time of such a treatment amounts to a number of hours, as is exemplified in greater detail below.
The time of heat treatment, exclusive of heating and cooling periods, is to be adjusted, of course, to the amount of oxygen in the gaseous atmosphere and to the heating temperature. It was found that this implies heating times in the range one half to ten hours, with optimum results at about three hours.
The surface layer thus obtained, with a thickness varying between 10.sup.-9 and 10.sup.-7 m, shows a structure and a composition which by degrees transforms from a condition in the outermost atom layer of the surface layer to a condition characterizing the matrix of the metal alloy.
The improved corrosion properties are attributed to an observed enrichment of alloying components, such as chromium, molybdenum or titanium in the surface layers thus formed.
As a consequence of the gradual transformation from substrate to surface layer, the surface layer is characterized by good adherence to the substrate. The transformation further takes place without great accumulations of defects or other material deficiencies. A further consequence of the fact that the surface layer is applied to the substrate internally and under vacuum conditions, is that the coat is the same all over the article surface, thus also in inward bulges and other surface areas where an external application would be rendered difficult by shading effects.
A surface layer made according to the present method on articles of iron-chromium alloys, which were heated in an oxygen containing gaseous atmosphere under low pressure, thus, comprises chromium of a higher concentration than the article in general, which chromium is oxidized entirely or partially, and which surface layer has a thickness of about 10.sup.-10 -10.sup.-6 m, preferably 10.sup.-9 -10.sup.-7 m.